Microsoft on Monday released an alert on two Active Directory vulnerabilities addressed with the November 2021 Patch Tuesday updates, urging customers to install the available patches as soon as possible, to prevent potential compromise.
Tracked as CVE-2021-42287 and CVE-2021-42278, the two security errors can be chained to impersonate domain controllers and gain administrative privileges on Active Directory.
Proof-of-concept code exploiting the two bugs has been public for more than a week, and Microsoft is warning companies of potential malicious attacks, while also sharing a guide to help organizations identify suspicious behavior exploiting the flaws.
“When combining these two vulnerabilities, an attacker can create a straightforward path to a Domain Admin user in an Active Directory environment that hasn’t applied these new updates. This escalation attack allows attackers to easily elevate their privilege to that of a Domain Admin once they compromise a regular user in the domain,” Microsoft explains.
CVE-2021-42278 is a security bypass vulnerability where attackers can employ computer account sAMAccountName spoofing to impersonate a domain controller.
sAMAccountName attributes, Microsoft notes, usually have “$” at the end of their names, which is meant to help distinguish between user and computer objects. Because of the bug, a normal user “has permission to modify a machine account (up to 10 machines) and as its owner, they also have the permissions to edit its sAMAccountName attribute,” Microsoft explains.
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Also a security bypass flaw, CVE-2021-42287 affects the Kerberos Privilege Attribute Certificate (PAC), also leading to domain controller impersonation. The vulnerability results in the Key Distribution Center (KDC) creating service tickets with higher privilege levels than those of the domain account.
During authentication using Kerberos, Ticket-Granting-Ticket (TGT) and the Ticket-Granting-Service (TGS) are requested from KDC. If the account for which TGS was requested could not be found, KDC would append a trailing $ to attempt to find it again.
“For example, if there is a domain controller with a SAM account name of DC1$, an attacker may create a new machine account and rename its SAM account name to DC1, request a TGT, rename it again for a different name, and request a TGS ticket, presenting the TGT he has in hands,” Microsoft notes.
Thus, with the lookup for DC1 failing, the KDC will attempt to find the machine trailing $, and “will issue the ticket using the privileges of DC1$.”
An attacker in the possession of domain user credentials can combine the two vulnerabilities and exploit them to gain domain admin privileges.
Microsoft has published a guide to help organizations identify any suspicious behavior related to the exploitation of these vulnerabilities by detecting abnormal device name changes and comparing them to a list of domain controllers in the target environment.
“As always, we strongly advise deploying the latest patches on the domain controllers as soon as possible,” Microsoft notes.
On November 14, the company released an out-of-band update to address a series of non-security issues related to applying the November 9 security updates.
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